Reaction between β-phenyl-α-alanine and 4-hydroxy-4-methylpent-2-ynenitrile: a spectroscopic and quantum chemical study

The sequence of isomeric transformations of intermediate adducts formed upon nuceleophilic addition of β-phenyl-α-alanine to 4-hydroxy-4-methylpent-2-ynenitrile affording a neutral or two alternative zwitterionic forms (with protonated imino or amino groups) of the product was studied quantum chemically by the B3LYP/6-31G(d,p) method and by UV spectroscopy. In the isolated state, the equilibrium is completely shifted toward the neutral form. Taking into account the polyhydrate environment increases the portion of the zwitterionic form bearing a protonated imino group of the dihydrofuran cycle. Protonation of the amino group of amino acid is thermodynamically unfavorable.